1 . The generation of internal stress
In injection molded products, the stress state varies locally and the degree of product deformation will depend on the stress distribution. These stresses can occur when the product experiences a temperature gradient while it is cooling and are therefore called “forming stress”.
There are two types of internal stress in injection molded products: molding stress and temperature stress.
When molten plastic material is injected into a mold at a lower temperature, the plastic material near the cavity wall quickly solidifies, causing the molecular chains to “freeze”. This results in poor thermal conductivity and a large temperature gradient across the thickness of the product. The core of the product solidifies more slowly, leading to a situation where the gate is closed before the melt has solidified in the center of the product. This prevents the injection molding machine from replenishing cooling shrinkage.
As a result, the interior of the product is under static stretching, while the surface layer is under static compression. The internal shrinkage of the product is opposite to that of the hard layer of the skin.
During the filling process, stress is not only caused by the volume contraction effect, but also by the expansion effect of the aisle and gate exit. The stress caused by the volumetric contraction effect is related to the flow direction of the molten plastic, while the stress caused by the expansion effect is perpendicular to the flow direction, due to the expansion at the outlet.
2. Process factors that affect stress
(1) Effect of directional stress
Under rapidly cooling conditions, orientation can cause internal stresses to form in the polymeric material. The high viscosity of the polymer melt means that the internal stress cannot relax quickly, which affects the physical properties and dimensional stability of the product.
Effects of parameters on orientation voltage:
- Melting temperature:
A high melting temperature leads to low viscosity and reduced shear stress, resulting in reduced orientation. However, high temperature also accelerates stress relaxation and improves orientation release. If the pressure of the injection molding machine is not adjusted, the cavity pressure will increase, leading to a stronger shearing effect and an increase in the guiding stress.
- Waiting time before nozzle closing:
Prolonging the retention time before the nozzle is closed increases the orientation tension.
- Injection and retention pressure:
Increasing injection or holding pressure increases orientation stress.
- Mold Temperature:
A high mold temperature ensures that the product cools down slowly, playing a disorienting role.
- Product thickness:
Increasing the product thickness reduces the orientation stress because thick-walled products cool slowly, leading to a slow increase in viscosity and a long stress relaxation process, resulting in a small orientation stress.
(2) Influence on thermal stress
As stated previously, the large temperature gradient between the melt and the mold wall during mold filling results in compressive stress (shrinkage stress) in the outer layer and tensile stress (orientation stress) in the inner layer.
If the mold is filled for a long period of time under the influence of holding pressure, the molten polymer is refilled into the cavity, increasing the cavity pressure and changing the internal stress caused by uneven temperature. However, if the retention time is short and the cavity pressure is low, the product will maintain its original state of tension during cooling.
If the cavity pressure is insufficient in the early stages of product cooling, the outer layer of the product will form a depression due to solidification contraction. If the cavity pressure is insufficient in the later stages, when the product has formed a hard, cold layer, the inner layer of the product may separate due to shrinkage or form a cavity.
Maintaining cavity pressure before gate closure helps increase product density and eliminate cooling temperature stress, but also causes a high stress concentration near the gate.
Therefore, when molding thermoplastic polymers, higher mold pressure and longer holding time help reduce shrinkage stress caused by temperature and increase compressive stress.
3. Relationship between internal stress and product is quality
The presence of internal stresses in a product can significantly affect its mechanical properties and usability. Uneven distribution of internal stress may cause the product to crack during use.
When used below the glass transition temperature, the product may suffer irregular deformations or warping, and its surface may become “white”, cloudy, with impaired optical properties.
Reducing the temperature at the port and increasing the slow cooling time can help improve the uneven stress on the product and make its mechanical properties more uniform.
Both crystalline and amorphous polymers exhibit anisotropic tensile strength. The tensile strength of amorphous polymers will vary depending on the location of the port. When the gate is aligned with the filling direction, the tensile strength decreases as the melt temperature increases. When the gate is perpendicular to the filling direction, the tensile strength increases with increasing melt temperature.
An increase in melting temperature strengthens the disorientation effect and reduces the orientation effect, reducing tensile strength. The orientation of the gate can affect the orientation by influencing the flow direction.
Amorphous polymers have higher tensile strength in the direction perpendicular to the flow direction than crystalline polymers due to their stronger anisotropy. Low-temperature injection has greater mechanical anisotropy than high-temperature injection, with the intensity ratio of the vertical direction to the flow direction being 2 when the injection temperature is low and 1.7 when it is high.
In conclusion, increasing melting temperature decreases tensile strength for both crystalline and amorphous polymers, but the mechanism differs, the latter being due to a reduction in orientation.
